People have always been fascinated with the measurement and properties of time. What happened to that last second that you just spent reading this sentence? Perhaps this is the reason that so many of us have tuned in over the years to the National Institute of Standards and Technology (NIST) “clock” operating at a
transmitting frequency of 10 MHz. The cesium standard NIST-7 has an accuracy of 5 ×10-15 seconds. Now that’s accurate! In fact, the definition of a “second” is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
We set our clocks, watches, and chronometers to the all-important “beep” of the radio signal that denotes the correct time to the second. In addition; avid listeners of the NIST clock are also treated to the following (from the NIST web site).
The OMEGA Navigation System status reports are voice announcements broadcast on WWV at 16 minutes after the hour. The OMEGA Navigation System consists of eight radio stations transmitting in the 10- to 14-kHz frequency band. These stations serve as international aids to navigation. The status reports are updated as necessary by the U.S. Coast Guard. Current geophysical alerts (Geoalerts) are broadcast in voice from WWV at 18 minutes after the hour. The messages are less than 45 s in length and are updated every 3 hours (typically at 0000, 0300, 0600, 0900, 1200, 1500, 1800, and 2100 UTC). Hourly updates are made when necessary.
Marine storm warnings are broadcast for the marine areas that the United States has warning responsibility for under international agreement. The storm warning
information is provided by the National Weather Service. Storm warnings for the Atlantic and eastern North Pacific are broadcast by voice on WWV at 8, 9, and 10 minutes after the hour. An additional segment (at 11 minutes after the hour on WWV) is used occasionally if there are unusually widespread storm conditions. The brief voice messages warn mariners of storm threats present in their areas. The storm warnings are based on the most recent forecasts. Updated forecasts are issued by the National Weather Service at 0500, 1100, 1700, and 2300 UTC for WWV. Since March 1990 the U.S. Coast Guard has sponsored two voice announcements per hour on WWV giving current status information about the GPS satellites and related operations. The 45-s announcements begin at 14 and 15 minutes after each hour.
WWV radiates 10,000 watts on 5, 10, and 15 MHz. The WWV antennas are half-wave dipoles that radiate omnidirectional patterns. The station uses double sideband amplitude modulation. The modulation level is 50 percent for the steady tones, 25 percent for the BCD time code, 100 percent for the seconds pulses and the minute and hour markers, and 75 percent for the voice announcements. That’s a lot of information being broadcast, and you can easily pickup these signals with the high frequency 10 MHz Time Receiver shown in this project. The WWV Time receiver has excellent sensitivity, selectivity, and dynamic range. (from the NIST web site http://www.nist.gov/)
Circuit description
The 10 MHz WWV Receiver is shown in the block diagram of Figure 10-3 and in the main schematic diagram in Figure 10-4. The WWV Time Code receiver is an AM TRF or Tuned Radio Frequency or TRF type receiver. This receiver project is designed specifically for 10 MHz time signal operation. The block diagram of the WWV receiver illustrates the four major sections of the receiver. The receiver’s front-end, as shown in the schematic diagram, features two sensitive dual-gate MOSFETs in the front-end circuit with a single-pole filter. An RF gain control is shown at VR1 which adjusts the input sensitivity, via a 10k ohm
potentiometer. Note inductor L1 is 4.1 µH coil, which consists of 27 turns of 22 ga. AWG wire wound on a
T-68 ferrite core. Inductors L2 and L4 consist of 12 turns of 22 ga. AWG wire on an FT37-43 ferrite core, and finally L3 consists of 25 turns of #26 ga. AWG wire on a T50-2 ferrite core. For even greater selectivity (but greater insertion loss), you can consider moving the L1 tap to 1 turn from ground. Fine tuning the WWV receiver is accomplished by adjusting CV1 and CV2 trimmer capacitors. The front-end MOSFETs are extremely sensitive to static electricity, so be very careful in handling them. Note the two ferrite beads, one is placed in series with the Drain lead of Q1, while the second ferrite bead is in series with the Drain of Q2. These ferrite beads are important, so be sure to
use them. The receiver front-end, at J1, should be connected to a dipole antenna “cut” for 10 MHz. TRF receivers are especially sensitive and are subject to overloading a BCB or Broadcast Band Interference, so internal lead lengths should be kept short between components and the front-end should be shielded and the receiver placed in a metal enclosure for best results.
The output impedance of Q2 is 2000 ohms, which nicely matches the input impedance of the Cohn crystal filter, in the crystal filter section of the receiver. This filter utilizes three 10 MHz crystals. Choose 10 MHz crystals that are marked for a 20 pF or 32 pF load capacitance if possible. Using a 10 MHz crystal oscillator, find three that are closest to one another in frequency. You may substitute 2.2k ohm resistors instead of the specified 2k with a slight penalty in pass band shape. The crystal filter section utilizes a single dual-gate MOSFET transistor at Q3. Coil L5 is 12 turns on an FT37-43 ferrite core. The output of the Cohn crystal filter is next fed to the input of the AM detector stage.
The AM detector has three distinct advantages; it has high bandwidth, low distortion and incredible (and variable) sensitivity. The variable bias control allows the listener to adjust the bias to maintain
detected audio fidelity even when the RF signal is weak. The detector uses a Schottky UHF mixer diode at D1. Increasing the diode bias from zero volts to maximum causes three things to happen: increased sensitivity, increased audio high frequency response, and a slight increase in receiver noise.
The AM detector section matches the 2000 ohm impedance of the crystal filter stage quite nicely. A sensitivity control at VR2 is a 1 megohm
potentiometer, which feeds directly to L6, a 1000 µH coil. Coil L6 is connected directly to a Schottky diode at D1. The resultant signal is next sent through a 5.6k ohm resistor and on to a 1 µF capacitor which feeds the nJFET at Q4. The Source lead of Q1 is connected to the Drain lead of Q5, and the Source lead of Q5 is
connected to ground via a 5.6k ohm resistor. Note the two 22 µF electrolytic capacitors on each of the Drain leads of the nJFETs.
When the WWV RF signal is weak, turning the bias off may result in the detected WWV signal
disappearing. Increasing the bias will bring the WWV signal back in. You run the bias control pot about half way and, of course, higher as WWV fades out. The fidelity that the bias adds even when the WWV signal is strong is quite pleasing to the ear. For the 1 µF and 2.2µF capacitors, I used polyester film types which sounded better than electrolytic capacitors.
The audio amplifier section features low noise audio amplifier. Distortion is very low as long as the input is
Figure 10-3 10 MHz WWV receiver block diagram